1. Field of the Invention
This invention pertains to a method of attaining a compressive stress of predetermined magnitude between a pair of contact surfaces including a device for doing so.
2. Prior Art
There are numerous examples where it has been desirable to know when a predetermined compressive stress has been attained in an assembly, for instance where a nut or bolt is used to hold an assembly together, or when a predetermined load is applied to a vehicle or lifting device. In the past, torque wrenches have been used to effect tightening, and it has been presumed that the torque would be a direct indication of the stress in the bolt. Such measurements are subject to considerable variation because the friction can vary due to surface finish, material hardness, and lubrication. Tightening tools of varying quality also produce a difference in results.
Various washers have been proposed to be used in a fastening device under a bolt head or nut to indicate a predetermined compression and hence tightness of the fastening device. These include a washer which changes color as a function of pressure, a laminated pad that transfers ink to successive layers as a function of pressure, a washer assembly which expands radially a predetermined amount as a function of pressure until a calibrated limit is indicated automatically, and a washer that has a compressible portion which on being compressed a given amount causes an inrease in torque needed to continue tightening, such increase being sensed.